Trilobites existed some 400 million years ago and extended in various forms, largely through the Ordovician, Cambrian and Devonian periods of geologic history. They were so successful at adapting to their environment, their populations exploded and there were massive swarms of them everywhere. They are now often referred to by paleontologists and marine biologists as the butterflies of the sea. It is surmised that Trilobites likely possessed the oldest preserved visual system [Clar 2006, Par 2003, pp 221] that we know of and actually contained a calcite eye lens.
In modern times, there is only one creature that has this distinction - the brittlestar. It's an aberration of nature, in a sense, that the large majority of species that came after the trilobyte developed protein crystalline form of eye lens – a much more efficient design.
The trilobite eye was an intricate arrangement of hexagonal lenses – similar to common insect eyes of today. At the time, sea water had a high calcite content, and this creature managed to find a way of putting the calcite crystals to work, where no-one else did. An illustrious career of numerous hundreds of millions of years speaks to the success of this rare biological innovation. Their lenses developed from an optically clear calcite form, which presently is used to polarize light in Nicol-Thompson prisms.
Evidence of calcitic eye lenses in trilobytes was determined in the fossil record due to the fact the calcite lens resisted decay and were present in well preserved fossils. In fact, the very detail that the lens was made of calcite helped to stay their preservation.
As a note of interest, optical calcite, or spar, was prized by Viking explorers who used it to help navigate the open seas.
#interesting facts about crystals